I absolutely suggest having this checked out. All too often, we fibromites blame everything on the fibro, and we must stop doing that! I narrowly avoided a hospital admission this week for that very reason. I was in a tremendous amount of pain from a wicked cold front with vicious winds, and didn't realize that I had developed a massive cellulitis infection in my leg. I went to prompt care primarily for pain relief and the infection was discovered then. I was put on massive amounts of antibiotics (which is another story, sigh). My son spent ten days in the hospital for the same thing just a few months ago.
Oh, and turns out some of the other severe pain I was having was likely from the infection, too. So get yourself to your doctor as soon as you can. There's a whole lot going on in your pelvic area--that will also cause back pain. Hope it's nothing serious, but better safe than sorry!